1. Field of the Invention
The present invention relates to the field of dynamic, impedance matched, transmission line terminations for common mode signals.
2. Background Art
Transmission lines play an important role in many fields of electronics and are particularly important in communications. Transmission line properties result from the geometrical relationships among conductors and ground structures and the properties of the conducting and insulating materials that form them. As a result, transmission lines are realized in various forms such as coaxial cables, twisted pairs of lines (both shielded and unshielded), microstrip and stripline structures. The quality of a transmission lines (the extent to which its performance approaches that of an ideal transmission line) can vary considerably and the choice is heavily influenced by the requirements of the specific application.
Discussion of the present invention will focus on an application with unshielded, twisted pair lines in a broadband communication application (digital subscriber line or DSL). This application reflects use of a “poor” quality line that is highly susceptible to both pick-up and sourcing of common mode noise signals. Of course these same twisted pair lines are “good” quality for their original design application—as a telephone line to transmit voice communication signals that are limited to under 4 KHz.
Referring to FIG. 1A, a typical transmission line is symbolically illustrated. Transmission line TL100 has its source or input port between nodes N101 and N102 and its terminal or output port between nodes N103 and N104. TL100 is fully symmetric with input and output ports. Voltage source V100 generates the input signal, which may be of arbitrary form. ZSOURCE is the termination impedance for the input port of TL100 and includes the source resistance of V100. ZLOAD is the termination impedance for the output port of TL100. The principal or defining electrical characteristic of a transmission line is its characteristic impedance (Z). For TL100 representing an ideal (lossless) transmission line with characteristic impedance Z, under the “matched” condition where ZSOURCE and ZLOAD are equal to Z, an input signal generated V100 will propagate undistorted to ZLOAD. Under this condition, half of the signal power will be dissipated in ZLOAD and half in ZSOURCE.
For the condition where ZLOAD does not equal Z, a portion of the propagating signal would be reflected back to the source where a portion of the reflected signal would be further reflected back toward the load if ZSOURCE also does not equal Z. FIG. 1B illustrates the condition in which ZLOAD is an open circuit, resulting in total reflection of the propagating signal power back to the source.
Clearly, any physically realizable transmission line will not be ideal and cannot be lossless. The propagating signal will therefore be attenuated and distorted as a function of frequency to at least some degree. Generally, these are characterized or modeled on a per-unit-length basis, but will clearly become more significant with increasing transmission line length.
The above discussion represents a very brief, non-mathematical summary of classical transmission line theory. What is not discussed in classical transmission line theory is the fact that the transmission line system illustrated in FIG. 1A, designed for transmission of differential mode signals produced by source V100, also represents a transmission line system for common mode signals that are picked up by the conductors. Since common mode signals are easily converted to differential mode and will be partially converted whenever they encounter an imbalance in impedance-to-ground, they can represent a significant source of signal distortion and noise, including crosstalk, to the desired signal propagation through transmission line TL100.
The common mode transmission line is comprised of the same physical structure as that for propagating the differential signals from source V100 to termination impedance ZLOAD. However, it represents one or more different type of transmission line structure with different electrical characteristics including characteristic impedance, signal velocity of propagation, and frequency characteristics. In some ways it can appear to function as a transmission line composed of a single conductor proximate to a ground plane. In other ways, it can appear to function as two of these types of lines in parallel. Creation of common mode transmission line models or mathematical descriptions of their operation for any of the numerous transmission line topologies is not the purpose of this teaching. The most significant fact is that no “matched” termination is provided the common mode, allowing these signals to reflect and bounce around until converted to harmful differential noise by circuit non-linearity or impedance imbalance. It would therefore be highly desirable to provide a matched transmission line termination for common mode signals that would not interfere with the normal operation of the transmission line in propagating differential mode signals.
FIG. 1C illustrates the generation, coupling, and transmission line propagation of common mode signals. Differential signal source V100 and impedance ZSOURCE are not shown to simplify the drawing. Source V101 is a ground referenced differential signal source. Source V101 would typically be one of many which would be distributed along the length of transmission line TL100. Sources injecting common mode at points other than terminal ports of transmission line TL100 cause common mode signals to propagate in both directions in said transmission line, implying the desirability of terminating common mode at both termination ports of transmission line TL100 (and even at any significant discontinuities existing between said ports). Source V101 typically would represent a noise source including signals generated and conducted on adjacent transmission lines, a noise source either internal or external to the system(s) of which transmission line TL100 is a part, and even signal source V100 which can source both differential and common mode signals into transmission line TL100.
Impedances ZCMSOURCE101 and ZCMSOURCE102 are the source impedance for the noise source to each line of transmission line TL100. They include the internal source impedance of source V101 that is common to both source impedances. Any mismatch between impedances ZCMSOURCE101 and ZCMSOURCE102 will result in proportionate conversion of the common mode signal to differential that will then propagate along transmission line TL100 with the desired signal from source V100. Since impedances ZCMSOURCE101 and ZCMSOURCE102 are not short or open circuits, they will provide some common mode signal termination at the point of noise injection but this is not likely to be even close to providing a match condition.
Finally, it should be noted that common mode signals present at nodes N103 and N104 produce no current flow or power dissipation within differential transmission line termination ZLOAD. For common mode signals, ZLOAD can be replaced with the open circuit shown in FIG. 11B without impact on common mode signals, again illustrating the need (in many applications) to provide a common mode termination for transmission line TL100.
FIG. 1D further illustrates the desirability to provide a true common mode, matched, transmission line termination rather than simply mitigate any imbalance in parasitic impedance from each line to ground. Such mitigation might be accomplished by adding precision, matched, “low” value resistors from each line to ground to swamp out the existent impedances. With this approach, the added resistors will also load the differential signal and must be accounted for in the overall circuit design. Mitigation might also be accomplished by adding a resistor to one line or the other to directly reduce but not eliminate the imbalance between the parasitic line impedances to ground.
In FIG. 1D, impedances Z103A and Z104A represent said parasitic impedances from each line to ground. ZN represents the impedance of a circuit branch coupling nodes N106 and N107 within the electronic circuitry using or associated with the output port signal of transmission line TL100. Impedances Z103B and Z104B are parasitic impedances coupling node N107 to nodes N103 and N104 respectively. Impedances Z103C and Z104C are parasitic impedances coupling node N106 to nodes N103 and N104 respectively. Impedances Z106A and Z107A respectively couple nodes N106 and N107 to ground. Impedances Z106A and Z107A provide the ground reference for common mode signal conversion and can represent parasitic capacitance, or actual components in the load circuit, or even zero if either node N106 or N107 is a ground connection. For conditions where there is an imbalance between impedances Z103B and Z104B or equivalently between impedances Z103C and Z104C (as there always will be to some extent), common mode signals are partially converted to differential signals that then appear across ZN, injecting noise directly into the circuitry. Presence of a common mode line termination coupled with proper blocking of common mode signals and isolation of the differential line termination can reduce this type of noise problem to relative insignificance.